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Tiny worm carries big burden

The mystery of why Colorado was so hard-hit by the scourge of whirling disease may be solved by a small aquatic worm known as tubifex tubifex.

Whirling disease is caused by a microscopic parasite that attacks the soft cartilage in young salmon and trout, including rainbow, brown and cutthroat trout.

The water-borne parasite (myxobolus cerebralis) may not directly kill trout, but highly infested fish can become deformed or exhibit the erratic tail-chasing behavior from which the disease gets its name.

The disease can wipe out entire generations of young fish.

The parasite’s life cycle needs two hosts — the young fish and a mud-dwelling worm.

But not just any worm, and not just any tubifex worm.

There are six known “lineages” of worm, but only lineage III can host the whirling disease parasite.

Unfortunately, the lineage III worm is dominant in Colorado.

“We don’t know why the lineage III worms predominate in Colorado,” said Sherman Hebein, senior fisheries biologist for the DOW Northwest Region, which includes Trappers Lake. “That’s why we were so badly hurt by whirling disease; other states don’t have the same mixture of lineages.”

Barry Nehring, an aquatics researcher with the Division of Wildlife, has been studying the life cycle of the tubifex worm for more than five years and said every place the lineage III tubifex is found, so is whirling disease.

In lakes and streams with other lineages of worms, the disease is absent.

“Lower Chicago Lake (on Mount Evans) is still negative for whirling disease 13 years after being stocked with WD-positive fish because no lineage III tubifex tubifex worms are present,” Nehring said.

Meanwhile, Upper Chicago Lake, stocked on the same day as the lower lake, is highly infected. And there Nehring has found lineage III worms.

One possible and admittedly remote chance to beat whirling disease is to replace the lineage III worms with other non-susceptible lineages. Test show that when worms become infected, they weaken and die, so replacing lineage III worms with lineage VI worms could halt the disease transmission.

“We’ve been trying all kinds of things with whirling disease,” said Sherman Hebein. “Barry’s research shows promise for culturing lineage VI worms and restocking them” where lineage III worms are found.